B How does the front wheel have no friction?

[per persson]

So I have difficulty understanding this https://physics.stackexchange.com/questions/331173/why-is-friction-only-on-the-back-wheel

With constant speed cycling uphill the front wheel has no friction. But if that would be the case how does the wheel even spin?

 

[PeroK]

So I have difficulty understanding this https://physics.stackexchange.com/questions/331173/why-is-friction-only-on-the-back-wheel

With constant speed cycling uphill the front wheel has no friction. But if that would be the case how does the wheel even spin?

At constant speed on a horizontal road there is effectively no friction on either wheel. The wheels continue to spin by conservation of angular momentum. When the bicycle changes speed, there must be friction between both tyres and the ground.

In the uphill scenario, at constant speed, there is friction on the rear tyre only.

 

[hutchphd]

Of course in the real world the bearings aren't perfect so there will be a very small friction force on the front wheel (backwards) from the ground and a very much larger frictional force forward on the back wheel from the ground to allow your legs to push you uphill.

 

[vela]

With constant speed cycling uphill the front wheel has no friction. But if that would be the case how does the wheel even spin?

You seem to have the common misconception that for an object to move or rotate, there has to be a force or torque acting on it. Newton's first law tells us that an object will continue with the same speed and same direction in the absence of any force acting on it. Similarly, the rotational analogue is that an object will continue to rotate in the absence of any torque acting on it.

With the bike moving with constant speed, no change in motion is required for the front wheel, so no net torque acts on the wheel. Since it's free-spinning, the only force that might produce said torque is friction; hence, no friction acts on the wheel.

The back wheel, in contrast, isn't free to spin, so it can provide the resistance needed to keep the bike from accelerating downhill. Therefore, friction acts through the back wheel to keep the bike moving up the hill at constant speed.

 

[sophiecentaur]

With constant speed cycling uphill the front wheel has no friction.

If only . . . . . .! If that were true then why wouldn't all bikes have knobbly tyres?
That rolling resistance is always with us. The contact surface of a wheel carrying a load is always climbing uphill as it moves over a surface, the distortion involves work / loss. Steel wheels on steel rails only distort a bit so that is a very efficient system.

The problem that people have is that they emotionally equate friction with loss. There is always some loss but the loss is often a small proportion of the power that's transferred by the 'friction' forces in a drive system.

 

[per persson]

At constant speed on a horizontal road there is effectively no friction on either wheel. The wheels continue to spin by conservation of angular momentum. When the bicycle changes speed, there must be friction between both tyres and the ground.

In the uphill scenario, at constant speed, there is friction on the rear tyre only.

I don't understand how there can be no friction at constant speed. Must there not be a force that counteracts the friction that tries to stop the bike. If there is no friction then a bike at constant speed can go forever no?

 

[PeroK]

I don't understand how there can be no friction at constant speed. Must there not be a force that counteracts the friction that tries to stop the bike.

If you have a bicycle, then try this. Hold the brakes and try to push the bike along the ground? It's very difficult to move at all. That's friction between the rubber tires and the ground. Now, release the brakes and the bike moves very easily. That's because there is no fiction for a rolling wheel.

However, there are three things resisting a moving bicycle: air resistance; friction within the moving parts (*); and, rolling resistance, where the tires lose energy through constant deformation (this is similar to friction, but the suggested experiment with the brakes, shows that it is very different from the friction between rubber and concrete).

(*) Note that this acts like a brake and, utimately, it's static friction with the ground that provides the exterbal force.

Note also that when the bike is moving and you apply the brakes, it's actually static friction (not kinetic friction) that provides the external braking force from the ground.

If there is no friction then a bike at constant speed can go forever no?

There are other resisting forces, other than friction. With no air resistance, a well-oiled bicycle will slow down very gradually (especially compared to a sliding object).

 

[jbriggs444]

Must there not be a force that counteracts the friction that tries to stop the bike. If there is no friction then a bike at constant speed can go forever no?

This sentence suggests that you have the picture shared by Aristotle. That objects in motion are only in motion because a force impels them to it.

You acknowledge the existence of a force of friction that tends to slow the bike down. But because the bike remains in motion, you imagine that some forward force must exist in order to keep pushing it forward.

It does not work that way. Newton's second law tells us how it actually works. "Inertia" is the $m$ in $F=ma$. It tells us how much force it will take to produce a particular acceleration. Friction is the only [horizontal] force acting on a bicycle that is slowing down.

Apologies if I have read into your sentence something that you did not mean to write.

 

[Lnewqban]

I don't understand how there can be no friction at constant speed. Must there not be a force that counteracts the friction that tries to stop the bike. If there is no friction then a bike at constant speed can go forever no?

In my opinion, following a straight line at constant forward speed, there is certain amount of rearward friction force from the pavement onto the contact patch of the front tire, which naturally tries to slow down by itself, while the chassis-rear wheel-legs force it to keep rolling.

If you flip your bicycle upside down and your hand gives some impulse to the front wheel, it will not rotate forever, because the moving rays suffer air drag that degrades its initial kinetic energy (besides bearing drag).
If you press a rolling cylinder against the tire (mimicking pressure of weight on asphalt), even more initial energy will be used to continuously deform the carcass and internal pliers.

If you go riding and encounter a curve, additional lateral friction will need to appear to compensate for the bike to bank and to remain turning (no lateral friction = no steering = no turning).

Finally, if you need to stop and apply only rear brake, the direction of the longitudinal force of friction (pavement onto rubber) will immediately reverse to forward, as the mass of the front will tend to keep rotating, while the chassis commands it to slowdown.

For a sprint, more rearward friction force will be needed to accelerate the rotation of the front tire.

 

[per persson]

This sentence suggests that you have the picture shared by Aristotle. That objects in motion are only in motion because a force impels them to it.

You acknowledge the existence of a force of friction that tends to slow the bike down. But because the bike remains in motion, you imagine that some forward force must exist in order to keep pushing it forward.

It does not work that way. Newton's second law tells us how it actually works. "Inertia" is the $m$ in $F=ma$. It tells us how much force it will take to produce a particular acceleration. Friction is the only [horizontal] force acting on a bicycle that is slowing down.

Apologies if I have read into your sentence something that you did not mean to write.

I think when the bike has constant speed the net force is 0 so it doesn't accelerate. If I cycle at constant speed and then stop cycling the bike will deaccelerate because of friction. If I want to keep it at constant speed I need to put in a force such that the horizontal net force is 0. If there is not friction at constant speed then why would I need to push the pedals?

I usually associate friction with motion. If an object moves on a surface then there will be friction with direction opposite of the motion of the object.

 

[russ_watters]

If there is not friction at constant speed then why would I need to push the pedals?

Note that the back wheel provides propulsion force but the front wheel does not. For thin tire bikes (road bikes) you only have rolling resistance, which is pretty small, especially as a fraction of propulsion force when climbing a hill.

I usually associate friction with motion. If an object moves on a surface then there will be friction with direction opposite of the motion of the object.

This is true for the idle wheel, but it is often very small. For the driven wheel the force is in the direction of motion.

 

[PeroK]

I think when the bike has constant speed the net force is 0 so it doesn't accelerate. If I cycle at constant speed and then stop cycling the bike will deaccelerate because of friction.

No. Because of air resistance.

If I want to keep it at constant speed I need to put in a force such that the horizontal net force is 0. If there is not friction at constant speed then why would I need to push the pedals?

There's air resistance.

I usually associate friction with motion. If an object moves on a surface then there will be friction with direction opposite of the motion of the object.

That's sliding, not rolling.

Note that friction against the motion of the bike will tend to speed up the rotation of the tires. Draw a diagram to see this. This is why it's rolling resistance (not friction) between the tites and the ground.

 

[sophiecentaur]

That's because there is no fiction for a rolling wheel.

. . . . except that this thread keeps hopping between ideal and real situations. There is no real wheel in this world that has no resistive forces between it and the surface it runs on. Aristotle's name crept in, higher up and all his (terrestrial) observations supported his statement. He could have looked in more detail at glass and metal rollers and noticed that his 'maintaining' force would depend on 'details'. But would you have done better without the influence of PF and others?

Is there seriously any point in trying to classify the contact forces under a wheel? You will never get rolling and no sliding or sliding and no rolling. Imo, you have to stand outside the situation if you want to 'explain' things to someone. If you want to have a proper answer to a proper 'ideal' question then you. must specify a tyre with no losses when it distorts, no slipping and no air. Plus, perhaps a horizontal surface. Then there's no energy added or no energy lost and the question becomes trivial. Starting there, you can add forces one at a time and see where it takes you.

 

[jbriggs444]

this thread keeps hopping between ideal and real situations.

I like to think about the "real" situation as myriad pieces relaxing toward an equilibrium. But modelling that would be unhelpful for a student struggling to grasp the basics. We deal with an emergent picture instead.

 

[hutchphd]

Then there's no energy added or no energy lost and the question becomes trivial. Starting there, you can add forces one at a time and see where it takes you.

I could not agree more.
The introduction of frictional forces early in the curriculum is (IMHO) a very confusing rubric When nonconservative forces exist, energy sometimes magically disappears into a cloud of dogmatic definitions: Dynamic friction, Static Friction, Rolling Fricion, Viscous Forces . Small wonder students are confused...I know I was.
How complicated is it to explain that the energy goes to degrees of freedom not explicitly considered part of the system and these can be internal or external?? I believe the overemphasis on various edsimple "friction"' is largely driven by the ease of creating compact exercises for the beginning student.

 

[russ_watters]

Based on the description in the OP I think there's only one possible answer to what gets included: gravity, pedal force and static friction.

 

[PeroK]

. . . . except that this thread keeps hopping between ideal and real situations. There is no real wheel in this world that has no resistive forces between it and the surface it runs on. Aristotle's name crept in, higher up and all his (terrestrial) observations supported his statement. He could have looked in more detail at glass and metal rollers and noticed that his 'maintaining' force would depend on 'details'. But would you have done better without the influence of PF and others?

Is there seriously any point in trying to classify the contact forces under a wheel? You will never get rolling and no sliding or sliding and no rolling. Imo, you have to stand outside the situation if you want to 'explain' things to someone. If you want to have a proper answer to a proper 'ideal' question then you. must specify a tyre with no losses when it distorts, no slipping and no air. Plus, perhaps a horizontal surface. Then there's no energy added or no energy lost and the question becomes trivial. Starting there, you can add forces one at a time and see where it takes you.

Nevertheless, you have to explain why you can push a bike easily with the brakes off, but with the brakes on it becomes almost immovable.

 

[hutchphd]

Nevertheless, you have to explain why you can push a bike easily with the brakes off, but with the brakes on it becomes almost immovable.

When the brakes are off they are not coupled to wheel rotation. When they are on there are other degrees of freedom which need to be included. ....?

 

[PeroK]

When the brakes are off they are not coupled to wheel rotation. When they are on there are other degrees of freedom which need to be included. ....?

But if there is friction in any case ...

This is the point of confusion: that a rolling wheel is subject to the same kinetic friction as a sliding wheel.

 

[Droidriven]

I can imagine pushing a car with half flat tires in neutral vs pushing a car with fully inflated tires in neutral. I can push with enough force to maintain a constant speed in both scenarios but in one it will be much easier, regardless of the fact that in both scenarios my feet are the point of friction for the force applied because I am pushing the car. Can we say that this indicates that there is definitely a non zero value of friction between the "freely" rolling tires with one scenario having more friction than the other?

Could it just maybe be that in one the friction is negligible enough that it's effect is virtually negligible in any kind of classical equation/solution you could ever need to apply to it and that we can "treat" it as zero for simplicity's sake?

 

[PeroK]

Could it just maybe be that in one the friction is negligible enough that it's effect is virtually negligible in any kind of classical equation/solution you could ever need to apply to it and that we can "treat" it as zero for simplicity's sake?

As long as you call it rolling resistance (or rolling friction) to distinguish it from sliding friction. If you simply call it friction, then you have the problem to explain why tires are made from the most frictional material (rubber) to "grip" the road.

In your analysis that makes no sense.

 

[Droidriven]

As long as you call it rolling resistance (or rolling friction) to distinguish it from sliding friction. If you simply call it friction, then you have the problem to explain why tires are made from the most frictional material (rubber) to "grip" the road.

In your analysis that makes no sense.

The friction I am referring to is the friction between the tire and the road at the surface area of contact. Whether the tire rolls or slides doesn't change the cause/source of the friction, it only changes the direction of force and the amount of friction. A sliding tire has more friction than a rolling tire but that doesn't mean the rolling tire has "no" resistance, just significantly less.

My proposed example was to demonstrate two scenarios where all variables are the same except for the amount of air in the tires/size of contact surface. Changing that single variable increases/decreases the friction between the tire and the road, demonstrating that the friction is definitely there, just significantly higher in one scenario than the other. This would be true even if the tire is made of a material that is less "grippy" than rubber, the amount of friction would just be less in both scenarios but still non zero.

 

[sophiecentaur]

As long as you call it rolling resistance (or rolling friction) to distinguish it from sliding friction. If you simply call it friction, then you have the problem to explain why tires are made from the most frictional material (rubber) to "grip" the road.

In your analysis that makes no sense.

Yes. @Droidriven is trying to use a model that's too simple; the old Classification rabbit hole again. Tyres are made the way they are because even the front tyres need to work also for braking and cornering. They exhibit both rolling and frictional 'resistance' in different amounts, according to function at the time and it's a compromise. Steel wheels on the railway are very low rolling resistance but rotten when you try to brake sharply.

Many vehicles have different wheel and tyre designs, front and back. I reckon cars would, too if there were any useful advantage (two spares needed???). But, of course, many cars have used different pressures, front and rear to give optimum cornering. (Softer front tyres can reduce over-steer, for instance)

 

[PeroK]

My proposed example was to demonstrate two scenarios where all variables are the same except for the amount of air in the tires/size of contact surface. Changing that single variable increases/decreases the friction between the tire and the road, demonstrating that the friction is definitely there, just significantly higher in one scenario than the other.

Except, it's not friction. It's rolling resistance. It's clear that rolling resistance is greater on soft tires. But, it's not clear that friction is greater on soft tires. Acceleration and braking are better on fully inflated tires, because the friction is greater. Or, at least, there is an optimum tire pressure.

PS for a given wheel/tire/vehicle we have: static friction, kinetic friction and rolling resistance. They are all different.

 

[Droidriven]

Except, it's not friction. It's rolling resistance. It's clear that rolling resistance is greater on soft tires. But, it's not clear that friction is greater on soft tires. Acceleration and braking are better on fully inflated tires, because the friction is greater. Or, at least, there is an optimum tire pressure.

PS for a given wheel/tire/vehicle we have: static friction, kinetic friction and rolling resistance. They are all different.

To say that a fully inflated tire has greater friction and acceleration goes completely counter to the drag racing world using lower tire pressure to increase surface area of traction(more friction between tire and track) to accelerate faster.

Rolling "resistance" and rolling "friction" are equivalent terms, in other words, it "is" friction, no, it isn't the same thing as static friction, but, it is friction nonetheless. In the OP, it is proposed that there is no friction on the freely rolling tire, as in, none at all. My posts are not to debate the specific type of friction at play but to demonstrate that there is definitely friction between a tire/wheel and a surface it is rolling on, whether freely rolling or not. Two independent objects in contact with each other+motion=equals some form of friction.

 

[jbriggs444]

Rolling "resistance" and rolling "friction" are equivalent terms, in other words, it "is" friction, no, it isn't the same thing as static friction, but, it is friction nonetheless.

A useful distinction can be made between "rolling resistance" and friction. Rolling resistance is more akin to a torque than to a resistance to linear motion.

In the simplest case, we are dealing with an increased normal force in the front of the contact patch where the tire is being deflected inward along with a reduced normal force in the trailing portion of the contact patch where the tire is returning to roundness. One can regard this either as a torque on the tire with the normal force of pavement on tire remaining centered or as an offset so that the normal force is not exactly centered under the axle. Neither interpretation directly involves any fore-and-aft frictional force. Both interpretations end up with a resulting torque on the tire.

A fore-and-aft frictional force can result.

If the wheel is powered, the power can directly counter the torque from rolling resistance so that the frictional effect is reduced, cancelled, reversed or [from the brakes or from engine braking] increased.

If the wheel is not powered, the effect of rolling resistance can be more easily seen. The result of an unbalanced net torque would be to slow the tire's rotation. But if the tire is rolling without [much] slipping then the pavement will prevent this. A linear frictional force between tire and pavement will arise. This force acts to preserve the rotation rate of the tire and to retard the linear motion of the vehicle.

It would be easy to call this portion of the linear frictional force between tire and pavement "rolling resistance". It is very easy to see how disagreements might arise over terminology even though there would be no disagreement about the physics.

 

[hutchphd]

For instance

A sliding tire has more friction than a rolling tire but that doesn't mean the rolling tire has "no" resistance, just significantly less.

Anyone who has driven an automobile on ice knows that this is not true. The answer is that the physics demands understanding and not just wasted rigid categoriazation. Perhaps one should expend mental energy on the former.

 

[A.T.]

Rolling "resistance" and rolling "friction" are equivalent terms, in other words, it "is" friction,

"Rolling resistance" is lumping together many resistive effects, which are not all frictional, but instead some come from the normal component of the contact force. Therefore calling it "friction" is a misnomer.

Aside of the asymmetrical normal force distribution mentioned by @jbriggs444 :

You can also have a deformation of the surface, which gives the normal force a horizontal component opposed to the linear motion.

Images from : https://en.wikipedia.org/wiki/Rolling_resistance